Recently, mobile information terminals such as a mobile phone or PDA (Personal Digital Assistance) have been required to be thinner and light in weight, and as a result, a speaker mounted in this mobile information terminal is required to be set in a small space.
Since a piezoelectric polymer material can be thinned into a film shape, it can be incorporated in the small space. In addition, a thin film-shaped piezoelectric film has flexibility and can show high piezoelectricity, so that a bimorph type or unimorph type piezoelectric element can be obtained by forming an electrode on a surface of the piezoelectric film. Therefore, the piezoelectric polymer material is also expected to serve as a speaker material for the mobile information terminal.
For example, Japanese Patent Laying-Open No. 2003-244792 (PTL 1) discloses a piezoelectric film speaker which includes a piezoelectric film diaphragm formed by transparent electrodes on front and back surfaces of a transparent and flexible piezoelectric film, and set on a display screen in a curved manner by supporting the piezoelectric film diaphragm on a casing of the mobile information terminal.
According to the PTL 1, a flexible piezoelectric film formed of polyvinylidene difluoride (hereinafter, referred to as “PVDF”) is used, so that the device can be low in profile and light in weight, the display screen can be large, and hearing performance is improved.
In addition, recently, in tandem with the development of a technique for a low-profile display, research and development have been increasingly carried out regarding a touch input system using a touch panel serving as an input interface of the mobile information terminal, mobile game machine, or mobile music player.
It is conventionally known that this kind of touch input system employs an ultrasonic method in which a surface acoustic wave technique is used, and a reflection array is formed along a side of a rectangular glass base material.
However, even when position information and press information on a plane are inputted with a finger or pen head at the same time according to the touch input system employing the ultrasonic method, there arises a problem that it is low in touch pressure resolution, poor in touch feel, and vulnerable to external impact because its surface is formed of the glass base material.
As opposed to the above, the piezoelectric polymer material has high piezoelectricity and flexibility, and it can be thinned into the film shape as described above, so that it is also expected to serve as the touch panel material.
In addition, Japanese Patent Laying-Open No. 5-61592 (PTL 2) discloses a touch input system configured by overlapping a flexible touch panel on a planar pressure sensitive sensor so as to adhere them to each other.
According to the PTL 2, a flexible piezoelectric film formed of polyester is arranged on the pressure sensitive sensor to obtain position information with the piezoelectric film, and obtain press information with the pressure sensitive sensor.